This invention relates to magnet materials for a high performance magnet for use in an electrical motor, a magnet sensor, a rotational sensor, an acceleration sensor and the torque sensor, etc., and more particularly to a bulk exchange-spring magnet with improved magnetic properties, a device incorporating the same and a method of fabricating the bulk exchange-spring magnet.
Permanent magnets have elementary magnetic dipoles that are aligned in one direction and that are not altered with outer magnetic fields. Thus, the permanent magnets have a large intrinsic saturation magnetization and exhibit a rectangular demagnetization curve to serve as an excellent material.
As the permanent magnets, a chemically stable ferrite magnet with low cost or a high performance rare earth magnet have been put to practical use in recent years. Among these permanent magnets which have been in practical use, however, even a neodymium magnet with the maximum performance has a limit in its intrinsic magnetic properties wherein the limit remains in the maximum energy product of 50 MGOe (about 4.0 MJ/m3).
The magnet has a structure that can be finely divided in a continuous step to finally result in atomic groups of Avogadro""s numbers. In such a scale of an intermediate level between a micro-scale and a macro-scale such as an original magnet structure, atomics cooperate with each other to produce a specific function. This intermediate region is termed as a mesoscopic domain or nanoscale domain. An exchange-spring magnet has been developed on the basis of a property peculiar to such a domain and has a hard phase (a permanent magnet phase) composed of a material having a high coercivity, and a soft phase (a soft magnetic material) composed of a material having a high magnetic flux density, which are magnetically coupled to one another through an exchange interaction to provide a magnet with a high maximum energy product.
In general, the structure of the exchange-spring magnet is composed of a plurality of laminated thin films of a hard and soft phase or of the soft phase composed of fine grains dispersed in basic structures of the hard phase, and is termed as a nanocomposite structure. The presence of the laminated structure of the thin films or the dispersed structure of the fine grains in a macrostructure results in mere coexistence of the hard phase and the soft phase in the magnet structure with a demagnetization curve, which represents the magnet properties, tracing a snake profile. When, however, the nanoscale domain is composed of the laminated structure or the grain dispersed structure, the magnetization of the hard phase is strongly restricted with the magnetization of the soft phase such that the nanoscale domain entirely behaves as it were a single hard phase. That is, when the exchange-spring magnet, wherein magnetization is aligned in one direction, is applied with the demagnetizing field in a negative direction, a reversal in magnetization occurs from an intermediate portion of the soft phase, with the magnetization, in the vicinity of the magnetic domain wall between the hard phase and the soft phase, remaining in its aligned condition in a positive direction owing to a strong exchange-force. Under such a condition, if the demagnetizing field is released, the magnetization returns along the demagnetization curve. Since this action is resembled to a spring action, the magnet is termed an exchange-spring magnet. Also, the word xe2x80x9cexchangexe2x80x9d is employed as an initial because its theory is based on an mutual exchange interaction.
For example, it is considered below about a strong magnetic composite wherein an axis of easy magnetization is oriented in one direction and the hard and soft phases are alternately laminated. When magnetically saturating the composite in a positive direction and subsequently applying the demagnetizing field to the composite in a negative direction, the magnetization is first reversed at the center of the soft phase. At the boundaries between the hard and soft phases, the magnetization of the soft phase is hard to be reversed because the orientation of the magnetization at the soft phase is restricted by the orientation of the magnetization of the hard phase owing to the exchange interaction with magnetic moment at the hard phase. While the magnetic moment at the hard phase may be slightly varied in orientation of the magnetization at the boundaries between the hard phase and the soft phase, the presence of the smaller magnetic field in the magnetization of the hard phase than that of the boundaries wherein the magnetization is irreversibly reversed allow the applied magnetic field to be returned to a zero state such that the system is subjected to a spring back to its original state. If the hard phase is applied with a greater magnetization than the magnetic field that is irreversibly reversed, the magnetization of the entire system is also irreversibly reversed such that the system is saturated in the negative direction.
In general, what the maximum energy product of the magnet is limited depends on the magnetization of the compound which functions as a main phase. The nanocomposite magnet has shown to theoretically surpass the limit of the performance of the magnet, which has been currently in practical use, such that the nanocomposite magnet surpasses the theoretical value of the maximum energy product of 120 MGOe (about 9.6 MJ/m3) of anistropic multi layers.
For all of these various reasons, the spotlight is focused on the exchange-spring magnet as a new magnetic material. The exchange-spring magnet has been usually developed mainly for the compound system composed of a hard phase containing a Ndxe2x80x94Fexe2x80x94B system or a Smxe2x80x94Fexe2x80x94N system and a soft phase containing Fexe2x80x94B or Fexe2x80x94Co compounds. Japanese Patent Provisional Publication No. 2000-208313 discloses a technology for obtaining an anistropic exchange-spring magnet powders in finer grains with superior magnetic properties by repeatedly implementing an amorphous processing step and a crystalline processing step.
As discussed above, the exchange-spring magnet theoretically tends to have the extremely high maximum energy product, though implementation of a full dense treatment of the exchange-spring magnet powders causes the exchange-spring magnet powders to be coarse in grain size at such a high sintering temperature of 1000xc2x0 C. required in the related art technologies, with resultant remarkably degraded magnetic properties (i.e., the maximum energy product). Therefore, it becomes difficult for the exchange-spring magnet powders to be densified in full dense state while maintaining the finer grain sizes of the magnet powders. Accordingly, in order to avoid the coarse grain growth, an extensive study has been conducted to apply the exchange-spring magnet powders to a so-called bonded magnet (in other word, a so-called plamag, plastic magnet or rubber magnet) wherein the magnet powders are mixed with plastic resin or rubber, followed by solidification of the magnet into a desired profile.
However, the density of the magnet powders contained in the bonded magnet remains at a remarkably lower value than the theoretical density of the magnet powders in the full dense magnet. This results in the formation of a final product with the maximum energy product which is far lower than that would be expected in the full dense magnet. That is, since the maximum energy product of the magnet decreases in proportion to the square of 2 of the charged density of the magnet, assuming that the charging rate is 50%, the energy product of the bonded magnet drops below 25% which is far lower than that of the bulk magnet.
The present invention has been made in view of the aforementioned circumstances and has an object of the present invention to provide an exchange-spring magnet having the density closest to the theoretical density of a full dense magnet powders shaped in a bulk without sacrificing the magnet properties.
It is another object of the present invention to provide a method of fabricating an exchange-spring magnet which has an improved magnet properties and which is enabled to be sintered at a lower temperature for a reduced time period than those of the related art technologies.
It is another object of the present invention to provide a device which employs an exchange-spring magnet with the aforementioned magnet properties, such as an electric motor, a magnet sensor, a rotational sensor, an acceleration sensor and a torque sensor.
According to a first aspect of the present invention, there is provided a bulk exchange-spring magnet which comprises a densified magnet powders including a hard phase and a soft phase, and boron atoms and oxygen atoms, wherein the boron atoms and the oxygen atoms cohere in boundary areas between grains of the densified magnet powders.
With such a structure, the presence of the cohering structure of the boron atoms and the oxygen atoms laying in the boundary areas between the grains of the magnet powders is effective for restricting the grain growth of the magnet powders. This results in a success of realizing superior magnet properties (i.e., maximum energy product) closest to that obtained in the theoretical density of the magnet powders of the full dense exchange-spring magnet.
According to a first aspect of the present invention, there is provided a bulk exchange-spring magnet which comprises means having grains formed of a hard magnetic phase and a soft magnetic phase mixed with one another, and means forming boundary areas between grains of the hard and soft magnetic phases to allow boron atoms and oxygen atoms to cohere therein.
According to a third aspect of the present invention, there is provided a method of producing a bulk exchange-spring magnet having densified magnet powders composed of a hard phase and a soft phase, and boron atoms and an oxygen atoms which cohere in boundary areas between grains of the densified magnet powders. The method comprises compacting magnet powders under a compacting pressure ranging from 300 to 1200 Mpa, heating the magnet powders under a compressed state at a starting temperature ranging from 25xc2x0 C. to a holding temperature ranging from 550 to 800xc2x0 C. at a temperature rising speed ranging from 5 to 40xc2x0 C./min, and holding the compressed magnet powders at the holding temperature for a time period of 0.01 to 10 min for thereby densifying the magnet powders.
According to a fourth aspect of the present invention, there is provided a method of producing a bulk exchange-spring magnet having densified magnet powders composed of a hard phase and a soft phase, and boron atoms and an oxygen atoms which cohere in boundary areas between grains of the densified magnet powders. The method comprises compacting magnet powders under a pressure ranging from 500 to 1200 Mpa, heating the magnet powders under a compacted state at a starting temperature ranging from 25xc2x0 C. to a holding temperature ranging from 650 to 700xc2x0 C. at a temperature rising speed ranging from 10 to 25xc2x0 C./min, and holding the compacted magnet powders at the holding temperature for a time period of 0.01 to 3 min for thereby densifying the magnet powders.
According to a fifth aspect of the present invention, there is provide a device equipped with a bulk exchange-spring magnet, wherein the bulk exchange-spring magnet comprises a densified magnet powders including a hard phase and a soft phase, and boron atoms and oxygen atoms, wherein the boron atoms and the oxygen atoms cohere in boundary areas between grains of the densified magnet powders.